1. Field of the Invention
The present invention relates to a method for regulation of the CO2 content in combustion products from a pressurised, non-adiabatic combustion chamber before expansion of the products of combustion to atmospheric pressure, an installation for execution of the method and a thermal power plant where the present method is used.
2. Description of Background Art
The concentration of CO2 in the atmosphere has increased by nearly 30% in the last 150 years. The concentration of methane has doubled and the concentration of nitrogen oxides has increased by about 15%. This has increased the atmospheric greenhouse effect, something which has resulted in:                The mean temperature near the earth's surface has increased by about 0.5° C. over the last one hundred years, with an accelerating trend in the last ten years.        Over the same period rainfall has increased by about 1%        The sea level has increased by 15 to 20 cm due to melting of glaciers and because water expands when heated up.        
Increasing discharges of greenhouse gases is expected to give continued changes in the climate. Temperature can increase by as much as 0.6 to 2.5° C. over the coming 50 years. Within the scientific community, it is generally agreed that increasing use of fossil fuels, with exponentially increasing discharges of CO2, has altered the natural CO2 balance in nature and is therefore the direct reason for this development.
It is important that action is taken immediately to stabilize the CO2 content of the atmosphere. This can be achieved if CO2 generated in a thermal power plant is collected and deposited safely. It is assumed that the collection represents three quarters of the total costs for the control of CO2 discharges to the atmosphere.
Thus, an energy efficient, cost efficient, robust and simple method for removal of a substantial part of CO2 from the discharge gas will be desirable to ease this situation. It will be a great advantage if the method can be realised in the near future without long-term research.
Discharge gas from thermal power plants typically contains 4 to 10% by volume of CO2, where the lowest values are typical for gas turbines, while the highest values are only reached in combustion chambers with cooling, for example, in production of steam.
There are three opportunities for stabilising the CO2 content in the atmosphere. In addition to the capturing of CO2, non-polluting energy sources such as biomass can be used, or very efficient power plants can be developed. The capturing of CO2 is the most cost efficient. Still, relatively little development work is carried out to capture CO2, the methods presented up till now are characterised either by low efficiency or by a need for much long-term and expensive development. All methods for capturing CO2 comprise one or more of the following principles:
Absorption of CO2. The exhaust gas from the combustion is brought into contact with an amine solution, at near atmospheric pressure. Some of the CO2 is absorbed in the amine solution which is then regenerated by heating. The main problem with this technology is that one operates with a low partial pressure of CO2, typically 0.04 bar, in the gas which shall be cleaned. The energy consumption becomes very high (about 3 times higher than if it is cleaned with a CO2 partial pressure of 1.5 bar). The cleaning plant becomes expensive and the degree of cleaning and size of the power plant are limiting factors. Therefore, the development work is concentrated on increasing the partial pressure of CO2. An alternative is that the exhaust gas is cooled down and re-circulated over the gas turbine. The effect of this is very limited due to the properties of the turbine, among other things. Another alternative is that the exhaust gas which is to be cooled down, is compressed, cooled down again, cleaned with, for example, an amine solution, heated up and expanded in a secondary gas turbine which drives the secondary compressor. In this way, the partial pressure of CO2 is raised, for example to 0.5 bar, and the cleaning becomes more efficient. An essential disadvantage is that the partial pressure of oxygen in the gas also becomes high, for example 1.5 bar, while amines typically degrade quickly at oxygen partial pressures above about 0.2 bar. In addition, costly extra equipment is required. Other combinations of primary and secondary power stations exist.
Air separation. By separating the air that goes into the combustion installation into oxygen and nitrogen, circulating CO2 can be used as a propellant gas in a power plant. Without nitrogen to dilute the CO2 formed, the CO2 in the exhaust gas will have a relatively high partial pressure, approximately up to 1 bar. Excess CO2 from the combustion can then be separated out relatively simply so that the installation for collection of CO2 can be simplified. However the total costs for such a system becomes relatively high, as one must have a substantial plant for production of oxygen in addition to the power plant. Production and combustion of pure oxygen represent considerable safety challenges, in addition to great demands on the material. This will also most likely require development of new turbines.
Conversion of the fuel. Hydrocarbon fuels are converted (reformed) to hydrogen and CO2 in pressurised processing units called reformers. The product from the reformers contains CO2 with a high partial pressure so that CO2 can be separated out and deposited or used in another way. Hydrogen is used as fuel. The total plant becomes complicated and expensive, as it comprises a hydrogen-generating plant and a power plant.
A common feature of the alternative methods for capture of CO2 from a power plant is that they strive for a high partial pressure of CO2 in the processing units where the cleaning is carried out. In addition, alternative methods are characterised by long-term, expensive and risky developments, with a typical time frame of 15 years research and a further 5 to 10 years or more before operating experience is attained. Expected electrical efficiency is up to 56 to 58% for a plant without cleaning and probably, somewhat optimistically, 45 to 50% with cleaning. An extended time frame is environmentally very undesirable. In a United Nations Economic Commission for Europe (UNECE) conference in the autumn of 2002, “an urgent need to address the continuing exponential rise in global CO2 emissions” was emphasised and words such as “as soon as possible” and “need to go far beyond Kyoto protocol targets” were used.
Thus there is a need for plants that overcome the mentioned problems, having the following characteristics:                Realisable without long-term development, preferably with the use of rotary equipment that has already been tested out.        Adapted for a sufficient partial pressure of CO2 so that conventional absorption installations can be used effectively, which means partial pressures up to 1.5 bar.        Lowest possible gas stream volume where CO2 shall be captured.        Partial pressure of oxygen down towards 0.2 bar where CO2 shall be captured for thereby to minimise the degradation of the absorption agent.        Possibility for effective cleaning of NOx, which is typically carried out in the temperature range 300 to 400° C. Cleaning in a pressurised system is optimal.        Efficiency in line with competing systems.        Possibility for large installations above 400 MW.        No use of secondary power systems, reformers, processes for production of oxygen or processes for conversion of the fuel.        Compact and robust plant to benefit from the cost advantages by building the plant at shipyards on floating constructions. This also makes use at offshore installations possible.        